An AC adapter is composed of a diode bridge circuit to rectify an alternating-current power supply, a DC-DC converter to lower a direct-current voltage rectified in the circuit to convert it into direct-current voltage of desired potential, and the like. As the DC-DC converter, for example, a switching power supply apparatus, in which current flowing through a primary winding of the voltage converting transformer is switching-controlled so that a voltage induced in a secondary winding is controlled, has been used.
Since miniaturization and low-cost production of the AC adapter have been pursued, it is important to reduce the number of mounting parts. For this reason, a control IC to control a switching transistor which applies current to the primary winding has been developed. Since a conventional switching control IC includes a relatively large number of outside elements such as a photo-coupler for feedback of an output voltage of the secondary winding to the control IC, capacitance and resistance, a circuit design for making the outside elements be incorporated in inside of the IC, and for reducing the number of the outside elements has been performed.
For example, as the invention relating to a switching regulator which need not have a photo-coupler or a secondary control IC, there is the invention disclosed in International publication number WO2004-082119 (Patent Document 1). FIG. 5 shows an entire configuration of the switching regulator disclosed in Patent Document 1, FIG. 6 shows a configuration example of a trigger control circuit which detects a terminal voltage of a primary auxiliary winding to provide a sampling timing, and FIG. 7 shows wave patterns of a signal and a voltage of inside of the regulator. Incidentally, the trigger circuit of FIG. 6 is placed in inside of the control IC 100 of FIG. 5.
In the switching regulator disclosed in Patent Document 1, as shown in FIG. 6, a change of a terminal voltage (hereinafter referred to as an auxiliary winding voltage) Vb of an auxiliary winding Nb is detected by comparators CP1, CP2 to generate signals Vg, Vd to control switches S1-S4 which charge and discharge capacitances C1, C2 are generated by a logic circuit including flip-flops FF1, FF2 and logic gates LG1, LG2 based on the signals Vg, Vd, and a timing when voltages of capacitances C1, C2 become equal is detected to generate sample hold signals S&H at the timing. The timing for generating the sample hold signals S&H is, as shown in FIG. 7, at a point Ps of two-thirds of voltage duration time Th of the auxiliary winding voltage Vb, and since the timing is near a point at which current Id flowing through a secondary diode or the secondary winding becomes zero, relatively high accuracy controlling can be performed.